The present invention relates to a catalyst for purifying hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx), which are contained in the exhaust gas from the internal-combustion engine of automobiles (of gasoline-type and of diesel-type) and boilers. The present invention relates in particular to a catalyst which is capable of purifying an oxygen rich exhaust gas by removing NOx contained in the oxygen rich exhaust gas.
In recent years, there has been a large demand for low-fuel-consumption automobiles, in view of the possibility of exhaustion of the petroleum resources and the global warming problems. In particular, there is a strong demand for lean-burn automobiles in the field of gasoline-engine automobiles. Such a lean-burn automobile is driven on the lean side with a large air-fuel ratio (A/F) as much as possible during an ordinary driving, so as to burn an oxygen rich air-fuel mixture and thereby meet with the demands for lower fuel consumption. In case of an oxygen rich (lean) air-fuel mixture, the oxygen content of an exhaust gas after combustion becomes high. If this exhaust gas is allowed to flow through a conventional three-way catalyst, the oxidation action becomes more active and the reduction action becomes inactive. In view of this, it has been desired to provide a catalyst which can sufficiently remove NOx by reducing NOx to N.sub.2 even on the lean side, that is, in an oxygen rich exhaust gas. For example, Japanese Patent Unexamined Publication JP-A-5-168860 discloses a catalyst having a porous carrier supporting thereon platinum and lanthanum. In more detail, this publication discloses a catalyst having a porous carrier (e.g., alumina) covered with lanthanum oxide, and platinum of the catalyst is supported on this lanthanum oxide. It is supposed in this publication that on the lean side (A/F=23) NOx is partly adsorbed on the lanthanum oxide of this catalyst and that under a condition with near the stoichiometric air-fuel ratio NOx which has been adsorbed thereon moves onto Pt (i.e., reverse spill over) and then is reduced to N.sub.2 by a reducing gas (CO, H.sub.2 and HC). European Patent Application 0589393A2 discloses a method for purifying an oxygen rich exhaust gas by using a catalyst comprised of (i) at least one noble metal selected from platinum and palladium (ii) barium, and (iii) at least one metal selected from alkali metals, iron, nickel, cobalt and magnesium. These components (i), (ii) and (iii) of this catalyst are supported on a carrier composed of a porous substance. JP-A-9-86928 discloses an A-site-deficient perovskite double oxide and a catalyst prepared by using this double oxide. This double oxide is represented by a general formula of A.sub.1-.alpha. BO.sub.3-.delta. where A is at least one selected from the group consisting of alkali metal elements, alkali earth elements, rare earth elements, Y, Bi and Pb, B is a 3d transition metal element and/or Al, .alpha. is greater than 0 and less than 0.2, and .delta. is from 0 to 1.